I've noticed that the Steelers usually generate a much more effective pass rush when they're standing up out of their stance, slightly off the line of scrimmage. They're getting a bigger head of steam, and so are able to push the offensive lineman back more effectively. Why doesn't the team employ this set more, especially on 3rd down? Am I overlooking a weakness of this set which would make Lebeau use it only sparingly?

When players are standing up, the offense doesn't know who's blitzing. Its to confuse them, when only 1 defensive linemen is down in a stance, they really aren't sure who's blitzing and who's not. Trust me I played center in a game against a zone blitz... most confusing thing when they are all standing up.

The reason they don't use it more is because the set also uses less defensive backs, so if the blitz would be picked up, there wouldn't be enough speed on the field to cover all the recievers.